Magnetic refrigeration utilizes so-called magnetocaloric effect in a magnetic material to perform the same task that a gas such as ammonia or freon does in a conventional refrigeration system. The first working magnetic refrigerator was put into use over fifty years ago to achieve ultra-low temperatures, the magnetocaloric effect being obtained in a paramagnetic salt. In 1976, the magnetocaloric effect in a ferromagnetic material near its Curie temperature was used to achieve magnetic refrigeration at higher temperatures.
Materials currently used for magnetic refrigeration fall into two categories:
(1) paramagnetic substances, e.g., Gd.sub.3 Ga.sub.5 O.sub.12, gadolinium gallium garnet, generally regarded as suitable for use at temperatures near and below 20.degree. K, and PA0 (2) ferromagnetic substances, e.g., Gd, for use at temperatures approaching room temperature. PA0 a first constituent, comprising a rare earth element; PA0 a second constituent, comprising a bulk component, PA0 wherein said first constituent is in the form of fine particles of nanometer size within a predetermined size range and is dispersed within said second constituent, PA0 whereby said first constituent provides finely distributed magnetic clusters within said second constituent in said nanocomposite material. PA0 a first heat exchanger; PA0 a second heat exchanger; PA0 a tubular element disposed with respect to said first and second heat exchangers to be able to perform heat transfer with each, said tubular element containing a heat exchange fluid to facilitate said heat transfers; PA0 a nanocomposite superparamagnetic element supported within said heat exchange fluid so as to be controllably moveable longitudinally within said tubular element between said first and second heat exchangers; and PA0 means for providing a controlled magnetic field extending into a region of said tubular element to which said nanocomposite superparamagnetic element is moveable to participate in heat transfer between said heat exchange fluid within said tubular element and said second heat exchanger. PA0 a first heat exchanger; PA0 a second heat exchanger; PA0 means for providing a controlled magnetic field that extends into a heat transfer zone within said second heat exchanger; and PA0 an element comprising a nanocomposite superparamagnetic material, supported to be controllably rotatable with a first portion of said magnetic element being in heat transfer communication with said first heat exchanger and a second portion of said magnetic element being in heat transfer communication with said second heat exchanger within said magnetic field, PA0 whereby rotation of said magnetic element causes movement of said first and second portions to and from said first and second heat exchangers to facilitate heat exchange therebetween.
In both cases, the heat transfer effected between two reservoirs at different temperatures is related to the magnetic spin entropy of the material. As an external magnetic field is adiabatically applied to a material, the magnetic moments of the constituent particles of the material try to align themselves with the external magnetic field, thereby reducing the magnetic entropy of the material and increasing its temperature. The opposite result, i.e., randomizing of the magnetic spins, increased spin entropy and decreased temperature, occurs upon the removal of the external magnetic field.
This "magnetocaloric effect" may be conveniently expressed as a temperature change dT experienced by the magnetic material and can be shown to be proportional to the change in applied magnet field strength dH, the temperature T, and the temperature dependence of the magnetization of the material at constant magnetic field strength, (dM/dT).sub.H. It is also inversely proportional to the material's heat capacity at constant field C.sub.H.
The magnetocaloric effect, thus defined, may be expressed as: EQU dT=-(T/C.sub.H).mu..sub.o V.sub.M (dM/dT).sub.H dH (1)
where .mu..sub.o is the magnetic permeability of free space, and V.sub.M is the volume of the material. If all the constituent magnetic moments were mutually aligned in the material, then M=N.mu., where N is the number of magnetic moments each of value .mu..
For a paramagnetic material at any temperature and field the individual magnetic moments are essentially random in orientation, and only partial alignment thereof is achievable by the imposition of an external magnetic field. See FIG. 1A. Hence, for a paramagnetic material which obeys the so-called Curie Law, EQU M=(N.mu..sup.2 H)/3k.sub.B T.
where k.sub.B is the Boltzmann constant. Consequently, the magnetocaloric effect for such a paramagnetic material may be expressed as follows: EQU dT.sub.para =(1/C.sub.H).mu..sub.o V.sub.M [N(.mu..sup.2)/(3k.sub.B)](H/T)dH (2)
For a ferromagnetic material below its Curie temperature T.sub.c, all the magnetic moments are aligned within domains. Consequently, the magnetocaloric effect is near zero for this material below its Curie temperature. See FIG. 1B. However, for a ferromagnetic material above T.sub.c M=(N.mu..sup.2 H)/[3k.sub.B (T-T.sub.C)], and its magnetocaloric effect is as follows: EQU dT.sub.ferro =(1/C.sub.H).mu..sub.o V.sub.M [N(.mu..sup.2)/(3k.sub.B)][HT/(T-T.sub.C).sup.2 ]dH (3)
At high temperatures, defined as T&gt;T.sub.C [HT/(T-T.sub.C).sup.2 ]&gt;(H/T) and dT.sub.ferro &gt;dT.sub.para which explains the usefulness of ferromagnetic materials for magnetic refrigeration at relatively high temperatures.
Currently, magnetic refrigerators are not practical for most commercial uses because dT is too small in magnitude. To obtain practical temperature differences, many stages must be used, making the apparatus complicated, expensive, and bulky. Practical magnetic refrigeration, therefore, requires improved magnetic materials, i.e. , affordable materials that exhibit a significantly high magnetocaloric effect.